Error diffusion methods, dither methods, and other techniques of halftone processing are well known in the art. Different methods of halftone processing produce different patterns in the output image, even when the same input image is subjected to the halftone processes. Therefore, a technology has been proposed for an image processor capable of executing a thresholding process based on an error diffusion method and a dither method that can ensure that colors rendered in the error diffusion method substantially match the colors rendered in the dither method. In order to make the output image produced by the dither method closely resemble the output image produced through the error diffusion method, this conventional technology sets threshold values for the dither matrix used in the dither method to conform with color tones of the image produced in the error diffusion method.